A gas absorption heat pump uses natural gas to drive an absorption cycle that moves heat from air, water, or ground into a building. It can deliver more heat than the gas it consumes, cutting fuel use versus furnaces and boilers. For many U.S. buildings with hydronic heating or high domestic hot water demand, a gas absorption heat pump can improve efficiency, reduce operating costs, and ease electric grid load while using a low‑GWP refrigerant. This guide explains technology basics, performance, applications, and what to consider before buying.
What Is A Gas Absorption Heat Pump?
A gas absorption heat pump (often shortened to GAHP) is a heating system that uses a fuel-fired burner, typically natural gas, to power an absorption refrigeration cycle. Instead of a compressor, it relies on chemical absorption and a small solution pump to move heat. The most common working pair is ammonia and water, with ammonia as refrigerant and water as absorbent.
Because it moves heat in addition to generating it from combustion, a gas absorption heat pump can achieve a heating coefficient of performance (COP) greater than 1.0. In practice, seasonal gas-based COPs of about 1.2–1.6 (on a lower heating value basis) are typical, depending on climate and system temperatures. This means 20–60% more useful heat per unit of gas than a 100% efficient baseline.
GAHPs are available as air-source units (air-to-water), water-source units (water-to-water), and ground-source units. Most U.S. residential and light commercial offerings are air-to-water configurations that integrate with hydronic distribution or produce domestic hot water.
Manufacturers market GAHPs under names like gas-fired heat pump, absorption heat pump, or absorption chiller-heater. While less common than electric heat pumps, they are proven in commercial buildings, multifamily properties, and facilities with constrained electrical capacity.
How A Gas Absorption Heat Pump Works
Absorption Cycle Basics
In a typical ammonia-water system, a gas burner heats a generator where ammonia vapor separates from the water solution. The high-pressure ammonia vapor flows to a condenser, rejects heat to the hydronic loop, and condenses to liquid. The liquid passes through an expansion device to low pressure and evaporates in an outdoor or source-side heat exchanger, absorbing heat from air, water, or ground.
The low-pressure ammonia vapor then enters an absorber, where water absorbs it and releases heat to the hydronic circuit. A small solution pump returns the rich ammonia-water mixture to the generator, completing the cycle. No mechanical compressor is needed, which reduces electric consumption and noise.
Some designs use advanced cycles, such as Generator-Absorber heat eXchanger (GAX), which recovers internal heat to boost efficiency. Control systems modulate burner input and fluid flow to match load and maintain supply temperatures, improving part-load performance.
Air-Source, Water-Source, And Ground-Source Variants
Air-source gas absorption heat pumps pull heat from outdoor air. They are simplest to install and common for retrofit hydronic systems. Performance improves in mild weather and drops in severe cold, though modern units operate at subzero temperatures with defrost strategies to manage frost on coils.
Water-source and ground-source GAHPs use water loops, geothermal borefields, or surface water as the heat source. These systems offer more stable source temperatures, yielding higher seasonal efficiency. They suit buildings with existing hydronic infrastructure or campuses where shared loops and thermal storage are practical.
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For domestic hot water or process loads, water-to-water absorption units can deliver higher outlet temperatures with good efficiency. Supply temperatures of 120–140°F are typical, and some models can produce up to 160°F under favorable conditions, aiding retrofits where radiators or air handlers need hotter water.
Reversible Heating And Cooling
Many gas absorption heat pumps are heating-only. Others are reversible or are packaged as absorption chiller-heaters. In cooling mode, the cycle produces chilled water by using the gas burner as the thermal input, similar to a direct-fired absorption chiller. Cooling COPs for gas-fired absorption are lower than electric chillers but can be compelling where electricity is costly or limited.
In mixed climates, reversible systems can meet year-round hydronic heating and cooling demands. When paired with thermal storage, they can shift gas consumption and reduce peak electric demand associated with cooling.
Efficiency And Performance Metrics
Efficiency for a gas absorption heat pump is usually expressed as a gas-based heating COP or Gas Utilization Efficiency (GUE). On a lower heating value (LHV) basis, seasonal heating COPs around 1.2–1.6 are common, with peak COPs in mild weather reaching 1.6–1.7. On a higher heating value (HHV) basis, these equate roughly to 1.1–1.5.
Compared with a 95% HHV condensing boiler, a GAHP with a 1.35 HHV seasonal COP reduces gas consumption by about 29% for the same delivered heat. For hydronic systems operating at lower supply temperatures (e.g., radiant floors at 100–120°F), efficiency improves further due to lower lift.
Ambient conditions strongly affect air-source GAHPs. Efficiency is highest at moderate outdoor temperatures and degrades as temperatures drop. Modern designs maintain operation to subzero conditions with capacity turndown and defrost cycles, but designers should account for reduced output on design days and consider buffer tanks or backup heat.
Supply water temperature is another key driver. Lower supply temperatures reduce the temperature lift across the absorption cycle, improving COP. When retrofitting high-temperature radiator systems, upgrading emitters, adding fan coils, or increasing surface area can unlock higher seasonal efficiency.
Electric consumption is minimal compared with electric heat pumps, as there is no compressor. Fans, pumps, and controls typically draw a few hundred watts. Total site energy use depends primarily on gas input, making GAHPs attractive where electricity tariffs are high or grid capacity is constrained.
Energy, Emissions, And Fuel Options
Because a gas absorption heat pump delivers more heat per unit of gas than a furnace or boiler, it can cut on-site fuel use and CO2 emissions. A GAHP at 1.35 HHV COP reduces combustion-related CO2 by roughly 29% compared with a 95% condensing furnace, assuming the same heating load and fuel.
When compared with electric air-source heat pumps, emissions depend on the local grid mix. In regions with low-carbon electricity, an electric heat pump usually has lower CO2 per unit of heat. In regions with higher-carbon grids or severe winter peaks, a GAHP can reduce both bills and peak electric demand while providing efficient heat.
Most GAHPs are designed for pipeline natural gas. Many burners can accept renewable natural gas (RNG) without modification, enabling substantial lifecycle emission reductions. Some units can operate with limited hydrogen blends; always confirm manufacturer limits for H2 percentage, materials compatibility, and warranty.
Ammonia, the primary refrigerant in many GAHPs, has zero ozone depletion potential and near-zero global warming potential. The ammonia charge is typically outdoors in packaged units. Proper siting, ventilation, and code compliance are essential due to ammonia’s toxicity.
Benefits And Trade-Offs
- Higher Fuel Efficiency: Delivers 20–60% more heat per unit of gas than 100% efficient heaters.
- Low Electrical Demand: No compressor; suitable for sites with limited electric capacity or high demand charges.
- Hydronic Compatibility: Pairs well with radiant floors, fan coils, and domestic hot water systems.
- Cold-Climate Operation: Functions in subzero temperatures; capacity decreases as ambient drops.
- Low-GWP Refrigerant: Ammonia systems avoid high-GWP HFCs.
- Quiet Operation: Fewer moving parts than compressor-based systems.
- Complexity: Absorption components and controls require trained service; fewer service providers than for boilers.
- Outdoor Space: Air-source units need exterior placement and clear airflow.
- Venting And Combustion: Requires gas piping, sealed combustion, and condensate management.
- Cooling Performance: Reversible absorption cooling efficiency is modest versus electric chillers.
Applications In Homes And Buildings
In single-family homes with hydronic heating, a gas absorption heat pump can replace or supplement a boiler, especially when supply temperatures can be kept at or below 130–140°F. It can also deliver high-efficiency domestic hot water with a buffer tank and appropriate controls.
Multifamily buildings benefit from centralized air-to-water or water-to-water GAHPs that serve multiple apartments via hydronic risers. The technology reduces common-area gas consumption and can integrate with existing radiators or fan coil units. Thermal storage enhances load matching and allows nighttime or off-peak operation.
Commercial facilities—schools, hotels, offices, and healthcare—can use GAHPs for space heating, service water heating, and process loads. In many cases, they pair with condensing boilers for peak loads, with the GAHP handling base load to maximize efficiency.
District energy and campus systems can deploy water-source or ground-source GAHPs on shared loops. These applications leverage diversity, heat recovery, and storage to raise system-level efficiency while keeping electrical demand modest.
Costs, Incentives, And Payback
Installed costs vary by size, configuration, and site conditions. For residential or light commercial air-to-water GAHPs, total installed cost commonly ranges from about $18,000 to $35,000, including hydronic integration and controls. Larger commercial systems, or ground- or water-source designs, can be significantly higher due to wells, loop fields, or plant room work.
Operating cost savings depend on relative gas and electricity prices, heating load, and achievable COP. Where gas is inexpensive and electricity rates or demand charges are high, payback can be favorable, especially when the GAHP displaces electric resistance, older boilers, or provides efficient hot water in high-use facilities.
Maintenance costs are typically modest but higher than a basic boiler due to periodic checks of the absorption solution and additional components. Budget for annual service by trained technicians and water treatment for hydronic loops to protect heat exchangers.
Incentives for gas absorption heat pumps are region-specific. While federal tax credits under recent energy legislation emphasize electric heat pumps, some U.S. gas utilities and state programs offer custom or prescriptive rebates for high-efficiency gas heat pumps, especially in commercial and multifamily settings. Consult your local utility and state energy office for current offerings.
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A simple payback example: Replacing a 95% boiler serving a 60 MMBtu/year load with a GAHP at 1.35 HHV COP reduces gas use by ~29%, saving ~17.4 MMBtu/year. At $10/MMBtu, that is about $174/year; at $15/MMBtu, ~$261/year. In larger or higher-load buildings, and when displacing electric water heating, savings scale materially.
Installation And Sizing Considerations
Proper sizing follows Manual J (residential) or ASHRAE load calculations (commercial). Designers should target loads at realistic supply temperatures. Oversizing offers limited benefit and can reduce efficiency; right-sizing with adequate hydronic buffer volume helps stabilize operation and defrost.
Distribution system compatibility is critical. Radiant floors and low-temperature fan coils maximize COP. For radiator systems, improving envelope insulation, adding emitter surface area, or using high-efficiency fan coils allows lower supply temperatures. Designing for 120–130°F supply is a common target to balance comfort and efficiency.
Air-source units require outdoor placement with clear intake and discharge paths. Avoid recirculation, snow drifts, and obstructions. Provide condensate drainage and ensure code-compliant gas piping and sealed venting. Noise is typically low, but maintain setbacks from windows and property lines as required locally.
Integration options include hybrid systems with condensing boilers for peak loads, domestic hot water preheat via buffer tanks, and controls that prioritize GAHP operation when conditions are favorable. Thermal storage can smooth defrost and align production with demand.
Maintenance, Reliability, And Safety
Annual maintenance should include burner inspection and tuning, vent and condensate checks, water treatment verification, and cleaning of outdoor coil surfaces for air-source units. The absorption solution and seals are part of a sealed circuit; service procedures follow manufacturer guidance and require trained personnel.
Reliability is strong when installed and maintained correctly. Fewer mechanical moving parts than compressor-based systems reduce wear. Controls and sensors are critical; robust commissioning and periodic calibration maintain performance.
Safety considerations include combustion safety and ammonia handling. Packaged GAHPs keep the ammonia charge outdoors or in mechanical spaces designed per code, with ventilation and leak detection as required. Adhere to local codes and standards for gas piping, venting, clearances, and refrigerant safety.
Low-NOx burners may be required in some jurisdictions. Verify compliance with regional air quality rules and ensure technicians are qualified for absorption systems, which differ from conventional boilers and electric heat pumps.
Comparison With Alternatives
| Technology | Typical Heating Efficiency | Grid Impact | Supply Temp | Strengths | Trade-Offs |
|---|---|---|---|---|---|
| Condensing Gas Boiler | ~0.90–0.97 HHV | Low Electric Use | Up To 180°F | Simple, Compact, Mature | Lower Efficiency Than GAHP, No Cooling |
| Gas Absorption Heat Pump | ~1.10–1.50 HHV COP | Very Low Electric Use | 120–160°F | Higher Fuel Efficiency, Hydronic Friendly | Outdoor Space, Specialized Service |
| Electric Air-Source Heat Pump | ~2.0–3.5 (Site COP) | Moderate–High Electric Use | 95–130°F Typical | Low Emissions On Clean Grids | Capacity Drops In Cold, Panel Upgrades |
| Ground-Source Heat Pump | ~3.0–5.0 (Site COP) | Moderate Electric Use | 100–130°F | High Efficiency, Stable Source | High First Cost, Drilling |
| Gas Engine-Driven Heat Pump | ~1.2–1.4 HHV Equivalent | Low Electric Use | Varies | High Capacity, VRF Options | Engine Maintenance, Noise |
The best choice depends on energy prices, grid constraints, climate, and distribution system. For buildings committed to gas service but seeking higher efficiency, a gas absorption heat pump can substantially reduce fuel use. Where electricity is low-carbon and available, an electric heat pump may minimize emissions.
Policy, Standards, And Buying Tips
Relevant standards include AHRI and ASHRAE test methods for absorption equipment and building codes governing combustion and refrigerant safety. Many GAHP products are tested under standards for absorption water heating and chiller-heater packages. Designers should request certified performance data, including seasonal COP at specified supply temperatures and ambient bins.
When evaluating proposals, insist on a detailed load analysis, hydronic design temperatures, expected seasonal efficiency (LHV and HHV), defrost strategy, and integration with domestic hot water. Ask for life-cycle cost analysis that includes maintenance and anticipated utility rates.
Useful resources include U.S. Department of Energy building technology guides, AHRI directories, and local utility program requirements. For general efficiency guidance, see DOE, ASHRAE, and AHRI.
Frequently Asked Questions
How Efficient Is A Gas Absorption Heat Pump Compared With A Boiler?
Seasonal heating COPs of about 1.2–1.6 LHV (roughly 1.1–1.5 HHV) are common, yielding 20–50% more delivered heat per unit of gas than a 95% boiler when distribution temperatures are moderate.
Can A Gas Absorption Heat Pump Work In Cold Climates?
Yes. Air-source GAHPs operate at subzero temperatures, though capacity and COP decline as ambient temperatures drop. Proper sizing, buffer tanks, and hybrid designs with a boiler can ensure comfort on design days.
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Does It Provide Domestic Hot Water?
Many GAHPs pair with buffer tanks to provide domestic hot water efficiently. Some water-to-water units are optimized for hot water production and can reach higher outlet temperatures suitable for commercial applications.
What About Maintenance?
Annual service is recommended: burner tuning, vent and condensate checks, hydronic water treatment, and coil cleaning. Absorption systems require trained technicians, but they have few mechanical wear components.
Is Ammonia Refrigerant Safe?
Ammonia is toxic but has zero GWP. Packaged GAHPs keep the ammonia circuit outdoors or in mechanical rooms per code with proper ventilation and, where required, leak detection. Follow local codes and manufacturer instructions.
Are There Incentives?
Federal incentives focus on electric heat pumps. Some utilities and states offer rebates for high-efficiency gas heat pumps, especially in commercial and multifamily settings. Check local programs for current terms and eligibility.
How Does A GAHP Affect Electric Bills?
Electric use is low because there is no compressor. Fans, pumps, and controls draw modest power, which can reduce demand charges compared with electric heat pumps, especially in large buildings.
Can It Use Renewable Natural Gas Or Hydrogen Blends?
Most units can use RNG. Limited hydrogen blends may be acceptable; verify manufacturer limits, combustion performance, and warranty conditions before using hydrogen-enriched fuels.
A Quick Checklist Before You Buy
- Heat Load And Temperatures: Confirm design loads and target supply temperatures at or below 130–140°F.
- Source Selection: Choose air-, water-, or ground-source based on site, performance, and budget.
- Hydronic Upgrades: Evaluate emitter upgrades or added surface area to reduce supply temperatures.
- Hybrid Strategy: Consider a condensing boiler for peaks; let the GAHP cover base load.
- Space And Siting: Ensure outdoor clearance, venting, drainage, and code-compliant gas piping.
- Controls And Storage: Include buffer tanks, outdoor reset, and smart staging or BMS integration.
- Service Capability: Identify trained technicians and set a preventive maintenance plan.
- Economics: Request a life-cycle cost analysis with utility rate projections and potential incentives.
Key Takeaways On Gas Absorption Heat Pumps
Gas absorption heat pumps deliver higher heating efficiency than furnaces and boilers by moving ambient heat with a gas-fired absorption cycle. They are well suited to hydronic systems, offer low electrical demand, and use low-GWP refrigerants. Success depends on thoughtful design: lower supply temperatures, proper sizing, and integration with storage and controls. In the right applications, GAHPs can cut fuel costs, reduce emissions versus standard gas equipment, and provide a practical path to efficient heating without major electrical upgrades.
How to Get the Best HVAC Prices
- Firstly, keep in mind that installation quality is always the most important thing for residential HVAC project. So never sacrifice contractor quality for a lower price.
- Secondly, remember to look up the latest rebates as we talked above.
- Thirdly, ask for at least 3 bids before you make the decision. You can click here to get 3 free estimates from your local contractors, and this estimate already takes rebates and tax credit into consideration and filter unqualified contractors automatically.
Lastly, once you chose the right contractor, remember to use the tactics from this guide: Homeowners Tactics When Negotiating with HVAC Dealer to get the final best price.
